Method of despreading GPS signals

ABSTRACT

A method of despreading GPS spread spectrum signals containing pseudorandom noise (PRN) code sequences and received by a GPS receiver ( 24 ) is disclosed together with a GPS receiver ( 24 ) and a mobile communications device (MS 1 ) (especially a mobile cellular telephone) for the same. The method comprises the steps of providing Doppler information relating to an estimate of the variation in Doppler shift as observed on the target signal by the GPS receiver and which is attributable to the motion of the GPS satellite; and correlating the target signal with a reference signal containing corresponding PRN code sequences, wherein, in the course of a single dwell, the correlation is modified as a function of the Doppler information.

This is a Continuation of application Ser. No. 10/067,366, filed Feb. 4,2002 now U.S. Pat No. 6,891,499.

FIELD OF INVENTION

This invention relates to a method of despreading a GPS spread spectrumsignal received by a GPS receiver, and to a GPS receiver and a mobilecommunications device (especially a mobile cellular telephone)incorporating such a GPS receiver for the same.

BACKGROUND TO INVENTION

It is well known to provide a GPS receiver in which replica GPSsatellite pseudorandom noise (PRN) code signals are continuous generatedand correlated with received GPS signals in order to acquire them.Typically, as the replica codes are likely to have a different codephase to those of the received GPS signals and also a differentfrequency due to Doppler shift between the receiver and orbitingsatellites, a two dimensional code frequency/phase sweep is employedwhereby such a sweep will eventually result in the incoming PRN codehaving the same frequency and code phase as that of the locallygenerated replica. If detected, the code is acquired and tracked, andthe pseudorange information may be retrieved from which the position ofthe receiver may be calculated using conventional navigation algorithms.

It is further known to provide a mobile cellular telephone incorporatingsuch a GPS receiver for the purpose of enabling operators of cellulartelephone networks to determine the location from which a call is madeand, in particular, for an emergency call to the emergency services. Ofcourse for an emergency call, it is desirable for the call location tobe available as soon as possible, however, from a “cold start” where theGPS receiver does not have access to up to date ephemeris data or evenworse from a “factory cold start” where the GPS receiver does not havean up to date almanac, the time to first fix (TTFF) can be anywherebetween 30 seconds and 5 minutes.

In order to reduce the TTFF, a GPS receiver may be provided with basestation assistance in order to acquire GPS signals more quickly. Suchassistance may include the provision by the base station to the receiverof a precision carrier frequency reference signal for calibrating thelocal oscillator used in the GPS receiver; the data message for up todate satellite almanac and ephemeris data from which Doppler shift forsatellites in view can be determined; and the current PRN code phase.With such assistance, it is possible to sweep only a narrowed range offrequencies and code phases in which the target PRN code is known tooccupy, thereby reducing the number of code instances that need to bechecked and thus reducing the time for code acquisition. Base stationassistance is further described in U.S. Pat. Nos. 5,841,396 and5,874,914 which are incorporated herein by reference.

A substantial reduction in the number of code instances that need to bechecked enables an increase in the dwell time for each check withoutsignificantly affecting the overall time to acquisition. The benefit ofthis is that an increase in the dwell time increases the probability ofacquiring weak GPS signals. For example, for a single code instance ordwell, correlation may occur over a period of 10 ms, equivalent toapproximately 10 PRN code repetitions (C/A mode) or over a period of 100ms consists of 10 incoherently summed individual correlation periods of10 ms.

OBJECT OF INVENTION

It is an object of the present invention to provide a method ofdespreading GPS spread spectrum signals received by a GPS receiver inwhich the probability of acquiring weak signals is increased and, inparticular but not exclusively, when dwells of extended duration areemployed to acquire such weak signals.

SUMMARY OF INVENTION

According to the present invention, such a method is provided comprisingthe steps of providing Doppler information relating to an estimate ofthe variation in Doppler shift as observed on the target signal by theGPS receiver and which is attributable to the motion of the GPSsatellite; and correlating the target signal with a reference signalcontaining PRN code sequences corresponding to those in the targetsignal, wherein, in the course of a single dwell, the correlation ismodified as a function of the Doppler information. This may occur, forexample, by modifying either the target signal or the reference signalas a function of the Doppler information prior to correlation.

By being aware of and compensating for such sources of error, itsdetrimental effect on the signal acquisition process can be avoided orat least mitigated, thereby aiding in the acquisition of weak GPSsignals. This is especially so when very long dwell periods are employedin an attempt to acquire remaining weak signals, for example, 1 s worthof summed individual 10 ms correlations. With a perfect frequency match,in theory one could integrate for an infinite period to acquire aninfinitely weak signal. However, the variation in Doppler shiftattributable to the motion of the satellite is typically be in the orderof 1 Hz per second which, without compensate such Doppler variation,limits the useful integration periods to around 0.5 s (that is ˜½ fwhere f is the frequency variation).

The Doppler shift may be calculated based on a last known position fixof the GPS receiver or alternatively, where the GPS receiver isincorporated in a mobile communications device adapted to communicatewith a nearby communications base station, based on a position fixprovided by the communications base station. For example, a position fixcorresponds to the location of the communications base station.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example only, ofan embodiment of a mobile cellular telephone comprising a GPS receiverfor use in a cellular telephone network with reference to theaccompanying schematic drawings in which:

FIG. 1 shows the geographic layout of a cellular telephone network;

FIG. 2 shows the mobile cellular telephone MS1 of FIG. 1 in greaterdetail;

FIG. 3 shows the base station BS1 of FIG. 1 in greater detail; and

FIG. 4 shows the GPS receiver and processor of the mobile cellulartelephone MS1 in greater detail.

FIG. 5 is a flow chart showing how Doppler information relating to anestimate of a variation in Doppler shift is used to modify a correlation

DETAILED DESCRIPTION

The geographical layout of a conventional cellular telephone network 1is shown schematically in FIG. 1. The network comprises a plurality ofbase stations BS of which seven, BS1 to BS7, are shown, situated atrespective, mutually spaced geographic locations. Each of these basestations comprises the entirety of a radio transmitter and receiveroperated by a trunking system controller at any one site or servicearea. The respective service areas SA1 to SA7 of these base stationsoverlap, as shown by the cross hatching, to collectively cover the wholeregion shown. The system may furthermore comprise a system controller SCprovided with a two-way communication link, CL1 to CL7 respectively, toeach base station BS1 to BS7. Each of these communication links may be,for example, a dedicated land-line. The system controller SC may,furthermore, be connected to a the public switched telephone network(PSTN) to enable communication to take place between a mobile cellulartelephone MS1 and a subscriber to that network. A plurality of mobilecellular telephones MS are provided of which three, MS1, MS2 and MS3 areshown, each being able to roam freely throughout the whole region, andindeed outside it.

Referring to FIG. 2, mobile cellular telephone MS1 is shown in greaterdetail comprising a communications transmitter (Comm Tx) and receiver(Comm Rx) 21 connected to a communications antenna 20 and controlled bya communications microprocessor (Comm μc) 22 for communication with thebase station BS1 with which it is registered. The design andmanufacturing of such telephones for two-way communication within acellular telephone network are well known, those parts which do not formpart of the present invention will not be elaborated upon here further.

In addition to the conventional components of a mobile telephone,telephone MS1 further comprises a GPS receiver (GPS Rx) 24 connected toa GPS antenna 23 and controlled by a GPS microprocessor (GPS μc) 25receiving GPS spread spectrum signals transmitted from orbiting GPSsatellites. When operative, the GPS receiver 24 may receive GPS signalsthrough an antenna 23 and pre-process them, typically by passivebandpass filtering in order to minimize out-of-band RF interference,preamplification, down conversion to an intermediate frequency (IF) andanalog to digital conversion. The resultant, digitised IF signal remainsmodulated, still containing all the information from the availablesatellites, and is fed into a memory of the GPS microprocessor 25. TheGPS signals may then be are acquired and tracked in any of severaldigital receiver channels, typically up to 12, for the purpose ofderiving pseudorange information from which the position of the mobiletelephone can be determined using conventional navigation algorithms.Such methods for GPS signal acquisition and tracking are well known, forexample, see chapter 4 (GPS satellite signal characteristics) & chapter5 (GPS satellite signal acquisition and tracking) of GPS Principles andApplications (Editor, Kaplan) ISBN 0-89006-793-7 Artech House. The GPSmicroprocessor 25 may be implemented in the form a general purposemicroprocessor, optionally common with the communications microprocessor22, or a microprocessor embedded in a GPS application specificintegrated circuit (ASIC).

Cellular telephone network base station BS1 is shown schematically inFIG. 3. In additional to the conventional components of a base station,it further comprises a GPS antenna 34, receiver 35 and microprocessor 36which are in substantially continual operation whereby the base stationis in constant possession of up to date GPS satellite information. Thisinformation includes which of the orbiting satellites are presently inview (such satellites are likely to be common to both telephone andassociated base station for even macrocells, obscuration aside); the GPSdata message containing an up to date almanac and ephemeris data andsatellite clock correction data, and the Doppler shift and current codephase of the GPS satellites signals as observed by the base station.

As is known, in the event of the user of the mobile cellular telephoneMS1 making an emergency call and under the control of the systemcontroller SC via a two-way communication link CL1, the base station BS1may provide this information to the telephone whereby it is then onlyrequired to sweep a narrowed range of frequencies and code phases inwhich the target PRN code is known to occupy, ensuring rapid codeacquisition and TTFF. A position fix then transmitted back to the basestation from the telephone, and then on to the emergency servicesoperator, termed the Public Safety Answer Point (PSAP) in the US.

Referring to FIG. 4, the GPS microprocessor 25 of the telephone MS1 isshown schematically implementing a pseudorandom noise (PRN) code sweepin which early (E), prompt (P) and late (L) replica codes of satellitePRN codes are continuously generated, and compared to the incomingsatellite PRN codes as received by the receiver. In order to retrievepseudorange information from the signal samples stored in the GPSmicroprocessor 25, a carrier wave must be removed and this is done bythe receiver generating in-phase (I) and quadrature phase (Q) replicacarrier wave signals using a carrier wave generator 41. A carrier wavephase lock loop (PLL) is normally employed to accurately replicate thefrequency of the received carrier wave. In order to acquire code phaselock, early (E), prompt (P) and late (L) replica codes of the PRNsequences are continuously generated by a code generator 42. The replicacodes are then correlated with the I and Q signals to produce threein-phase correlation components (I_(E), I_(L), I_(P)) and threequadrature phase correlation components (Q_(E), Q_(L), Q_(P)), typicallyby integration in an integrator 43. A code phase discriminator iscalculated as a function of the correlation components and a thresholdtest applied to the code phase discriminator; a phase match is declaredif the code phase discriminator is high and if not, the code generatorproduces the next series of replicas with a phase shift. A linear phasesweep will eventually result in the incoming PRN code being in phasewith that of the locally generated replica and thus code acquisition.

In accordance with the present invention, the GPS processor 25 of mobiletelephone MS1 may acquire incoming GPS signals as illustrated in thefollowing example and FIG. 5,

EXAMPLE

A user of mobile cellular telephone MS1 located inside a building whereGPS signal reception is generally poor makes an emergency call to theemergency services (termed “public safety answer point” in the US).Under the control of the system controller SC via a two-waycommunication link CL1, the base station BS1 provides up to date almanacand ephemeris data, and the Doppler shift of the GPS satellites signalsas currently being observed by the base station.

The GPS receiver samples 100 ms of GPS signals and then, using thesatellite information provided by the base station, the GPS processor 25employ a conventional early-minus-late correlation architecture in anattempt to acquire the GPS signals. Using a 10 ms portion of the 100 msworth of GPS signal sampled, the GPS processor 25 sweeps only a narrowedrange of frequencies in which the target PRN code is known to occupy andin doing so manage to acquire two GPS signals having a relatively strongsignal-to-noise ratio. This may occur where, for example, the respectiveGPS satellites are in direct view of the GPS receiver though windows inthe building. Then, having completed an unsuccessful sweep for theremaining GPS signals, two further being required to obtain a positionfix, the GPS receiver employs a modified acquisition process in which:

(1) Using one of the signals currently acquired, the GPS processor 25measures the variation in frequency of that signal as observed by theGPS receiver throughout the 100 ms GPS signal sample. This may be doneby either repetitively acquiring that signal using several 10 ms dwellsthroughout the 100 ms sample sequence; or having acquired that signalusing an initial 10 ms part of the 100 ms sample sequence, tracking thatsignal though the 100 ms sample sequence. The variations are typicallyattributable to local oscillator drift, the reference to which thefrequencies are measured by the GPS receiver, and variations in Dopplershift attributable to both handset and satellite movement.

(2) The frequency variation profile may be modified to exclude thosefrequency variations attributable to Doppler shift caused by themovement of the satellite associated with the acquired signal which canbe readily calculated from empheris data provided by the base station orfrom a previously acquired GPS signal, a position estimate such as onebased on a last known position fix or a position fix provided by thecommunications base station, and a knowledge of GPS time which may bederived from one GPS satellite and a position fix estimate.

(3) To assist in the acquisition of a further GPS signal, the frequencyvariation profile may be further modified to compensate for expectedfrequency variations attributable to Doppler shift cause by the movementof the satellite associated with that signal, i.e. the target signal.Again, this may be readily calculated from empheris data provided by thebase station or acquired from a GPS signal, a position estimate andknowledge of GPS time.

(4) Using a dwell over the whole 100 ms worth of GPS signal samples, theGPS processor 25 again sweeps only a narrowed range of frequencies inwhich a target PRN code is known to occupy. This time however, thecorrelation process employed to acquire that signal is modified inaccordance with the frequency variation profile as modified after step(3). That is, the effects of handset movement and local oscillator driftare removed or at least mitigated. This is done in any of the followingways: prior to processing the data in a conventional manner, mixing itwith a signal that represents the detected frequency variation; orinstead of mixing the data with a fixed frequency offset signal as partof the conventional search mechanism, using a variable frequency signal,adjusted in such a way as to incorporate the measured frequencyvariation.

In the above example, the variation in Doppler is taken into accountduring acquisition of relatively weak signals after 2 relatively strongsignals have been acquired. Such variation may also be taken intoaccount in order to acquire a first signal when, for example, ephemerisdata, an estimate of the position of the mobile telephone and currentGPS time is provided to the cellular telephone by a base station.

Furthermore, as an alternative to the early-late correlation method,fast convolution methods and in particular, involving Fast FourierTransforms (FFTs), may be used in order to acquired the PRN codes. Suchconvolution methods are described in a paper entitled “FFT processing ofdirect sequence spreading codes using modern DSP microprocessors” byRobert G Davenport, IEEE 1991 National Aerospace and ElectronicsConference NAECON 1991, volume 1, pages 98 to 105, and also in U.S. Pat.No. 5,663,734. The method of the present invention is equally isapplicable to such convolution methods at least in that any carriercould be stripped from the signal as described above, before the FFTconvolution was carried out.

The invention has largely been described in the context of NAVSTAR GPS,the all weather, spaced based navigation system developed and currentlyoperated by the US Department of Defense. However, it will beappreciated that the general underlying principles of GPS are universaland not merely limited to NAVSTAR. Accordingly, GPS is intended to referto any positioning system comprising a plurality of spread spectrumradio transmitters at different locations and a receiver whichdetermines its location based on the time of arrival of thetransmissions of the radio transmitters.

From a reading of the present disclosure, other modifications will beapparent to the skilled person and may involve other features which arealready known in the design, manufacture and use of GPS receivers andcomponent parts thereof and which may be used instead of or in additionto features already described herein. Although claims have beenformulated in this application to particular combinations of features,it should be understood that the scope of the disclosure of the presentapplication also includes any novel feature or any novel combination offeatures disclosed herein either explicitly or implicitly, whether ornot it relates to the same invention as presently claimed in any claimand whether or not it mitigates any or all of the same problems as doesthe present invention. The applicants hereby give notice that new claimsmay be formulated to such features and/or combinations of such featuresduring the prosecution of the present application or of any furtherapplication derived therefrom.

1. A method of despreading a target GPS spread spectrum signalcontaining pseudorandom noise (PRN) code sequences and received by a GPSreceiver, the method comprising the steps of: providing Dopplerinformation relating to an estimate of the variation in Doppler shift asobserved on the target signal received by the GPS receiver and which isattributable to the motion of the GPS satellite; correlating the targetsignal with a reference signal containing corresponding PRN codesequences, and in the midst of a single dwell, modifying the correlationas a function of the Doppler information to compensate for the variationin Doppler shift in the course of the dwell.
 2. A method according toclaim 1 wherein the target signal is modified as a function of theDoppler information prior to comparing it with the reference signal. 3.A method according to claim 1 wherein the estimate of the variation inDoppler shift is calculated based on a last known position fix of theGPS receiver.
 4. A method according to claim 1 wherein the GPS receiveris incorporated in a mobile communications device adapted to communicatewith a nearby communications base station; and wherein the estimate ofthe variation in Doppler shift is calculated based on a position fixprovided by the communications base station.
 5. A method according toclaim 4 wherein the position fix corresponds to the location of thecommunications base station.
 6. A GPS receiver for despreading a GPSspread spectrum signal received by the GPS receiver comprising: meansfor providing Doppler information relating to an estimate of thevariation in Doppler shift as observed on the target signal by the GPSreceiver and which is attributable to the motion of the GPS satellite;means for correlating the target signal with a reference signalcontaining corresponding PRN code sequences, and means for, in the midstof a single dwell, modifying the correlation as a function of theDoppler information to compensate for the variation in Doppler shift inthe course of the dwell.
 7. A mobile telephone comprising a GPS receivercomprising: means for providing Doppler information relating to anestimate of the variation in Doppler shift as observed on the targetsignal by the GPS receiver and which is attributable to the motion ofthe GPS satellite; means for correlating the target signal with areference signal containing corresponding PRN code sequences, and meansfor, in the midst of a single dwell, modifying the correlation as afunction of the Doppler information to compensate for the variation inDoppler shift in the course of the dwell.